1. Field of the Invention
The invention pertains to the field of high-density electronic packaging. More particularly, the invention pertains to electrical contacts for high-density connectors, interposers and chip-level packaging.
2. Description of Related Art
High-density electronic package can be designed in two ways separately or in combination. One of the ways is to make the size of or the area occupied by the package not much larger than that of the chip or die in the package. This approach allows the highest package density in 2-D therefore the device density in a given system space. Chip-scale package is an example of such an approach.
The other way or approach is to pack as many dies or devices as possible in a package. A way to do this is to stack the chips or dies in a package while maintaining the area occupied by the package about the same.
Chip-scale package with stacked chips or dies is an example of high-density package that combines the above two approaches at the same time.
To meet the desire of achieving high device or package density further one can stack the packages together while keeping the area occupied about the same.
The sub-packages with stacked dies and stacked sub-packages can also be packed together in a package either arranged in 2-D or 3-D in a system-in or on-package approach to achieve even more packing density.
In current practice the die or chip is attached in the above by either wire bonding or flip-chip solder joint. Thus the design of the above packages will differ depending on the method of die-attach.
Currently the micro-BGA (Ball Grid Array) external connection of a chip-scale package has a limited number of I/O (about 200 or less) as the result of lack of thermal-mechanical reliability. The wire bonding commonly used for die attaches whether for a single die or stacked dies in a chip-scale package suffers from being an inductive electrical connection with limited electrical conductance.
Whether one uses wire bonding or flip-chip solder joint for die-attach they are not reworkable. The BGA or micro-BGA solder joint commonly used for package stacking is not reworkable. Thus costly known-good-die and known-good-package are necessary in the above approaches to achieve acceptable yield especially in a system-in-package approach.
In prior art high-density packages there is not any convenient way to cool a die in a stacked structure or to attach a heat spreader to a die in the same. Thus chips such a microprocessor that requires high heat dissipation capacities cannot be packaged easily in a stacked package whether for die or package stacking.
There is therefore a desire to use demountable connections in die-attach and in package stacking to enhance yield and reduce cost in high-density packages. The demountable connections such as UEC offer also high performance in terms of frequency and current, high service temperatures, as well as larger I/O arrays at finer contact pitches.
There is a desire to solder electrical contact such as UEC at both ends or wire-bond (thermal-sonic-compression-bond) at one end and solder at the other end to achieve a small stack height of a package. Such connections allow test first followed by assembly for high yield.
There is also a desire to be able to provide high capacity heat spreader attached to dies in a stacked structure of high-density packages.
FIG. 1 depicts a typical prior art chip-scale or high-density package with a single chip or a stack of chips, in this case two chips (11) and (12). The chips are connected by wire bonds (14) and (15) from the bonding pads on the chip to the bonding pads (16) and (17) on the substrate (10) of the package. The substrate (10) is usually made of epoxy/glass composite. A number of ways have been used in the prior art for connecting or routing the bonding pads (16) and (17) on the backside of the substrate (10) of the package to the contact pads (13) on its front face to achieve high-density or chip-scale packaging.
The contact pads (13) on the front face of the package are typically connected to the contact pads on the main board by micro-ball-grid-array or micro-BGA solder joints (18).
The prior art shown in FIG. 1 suffers from:                Inductive long bonding wires (14) and (15).        Lack of heat spreading or dissipation capabilities.        Limited number of I/O allowed by micro-BGA solder joint external connection to avoid thermal expansion coefficient mismatch failure.        
The use of the prior art in FIG. 1 is primarily concentrated in memory chip packaging where the I/O number is low and heat dissipation requirements are limited.
FIG. 2 shows a typical prior art package using stacked chip-scale or high-density sub-packages. In the version shown in FIG. 2, there are two sub-packages, an upper sub-package (29) and a lower sub-package (30). The upper sub-package (29) is essentially the same as a single-chip embodiment of the design of FIG. 1, with a chip (21) on a substrate (31), having wire bonds (25) connecting the chip (21) to contact pads (23) on the upper surface of the substrate (31). A second chip (22) is located on the lower sub-package (30), bonded by wires (24) to contact pads (26) on top of the substrate (32) of the lower sub-package (30).
The stacking of the sub-packages is achieved by connecting electrically the contact pads (27) on the lower face of the upper sub-package (29) to the contact pads (28) on the upper face of the lower sub-package (30) by ball-grid-array or BGA solder joint or micro-BGA solder joint. To avoid height interference the chip (22) may sit in a cavity in the substrate (32) of the lower sub-package (30).
The external contact pads (13) of the package are connected to the same on the main board by BGA or micro-BGA solder joint (18), as in FIG. 1. The stacked packages of FIG. 2, although achieving higher density, do not avoid the limitations of the package in FIG. 1.
FIGS. 3 and 4 show prior art examples of system-in-package designs where the packages in FIGS. 1 and 2 are packaged in combination in 2-D and/or 3-D. The methods of connection used in such a complex package include wire bonding, BGA and micro-BGA. These methods all provide permanent connections. Thus known-good-die (KGD) and known-good-package (KGP) testing methods are highly desirable here to increase yield and reduce cost. It should be recognizes that KGD and KGP carry extra testing cost in themselves.
FIG. 5 depicts a prior art single chip chip-scale or high-density package in which chip (52) attach is made by using flip-chip solder joint (50) between the chip (52) and the contact pads (51) on the upper surface of the substrate (54) of the package.
Flip-chip solder joint without under-fill protection from thermal expansion coefficient mismatch damages suffers also from low I/O numbers. A ceramic substrate of the package may be used for low thermal expansion coefficient mismatch, but this approach is costly.
Currently flip-chip solder joint has not been used commercially for chip stacking in a high-density package. In a system-in-package design a mixture of die attach by wire bonding and by flip-chip solder joint are often seen.
FIG. 6 shows three high performance demountable electrical contacts that are suitable for electrical connection in high-density electronic package of the present invention. These connectors will be referred to as the “Universal Electrical Contact” (61) hereinafter “UEC”), “Torsion-Spring contact” (62) (hereinafter “T-Spring”), and “Folded-Spring contact” (63) (hereinafter “F-Spring”). The electrical contact UEC is described in four prior US patents granted to the present inventor: U.S. Pat. No. 7,040,902, “Electrical Contact”, and three Continuations-in-Part—U.S. Pat. No. 7,014,479, “Electrical Contact and Connector and Method of Manufacture”, U.S. Pat. No. 7,029,288, “Electrical Contact and Connector and Method of Manufacture”, and U.S. Pat. No. 7,029,289. The T-Spring is described in co-pending U.S. patent application Ser. No. 11/264,803, entitled “Electrical Contact Assembly and Connector System”, filed Nov. 1, 2005, and the F-Spring is described in co-pending U.S. patent application Ser. No. 11/334,993, entitled “Electrical Contact and Connector System”, filed Jan. 18, 2006. The contents of the aforementioned patents and patent applications are incorporated here by reference.
The UEC (61) has a multiple conductive wire braided cylindrical structure as shown in FIG. 6. The T-Spring (62) is made of multiple conductive one-turn torsion springs (64) operating in parallel mounted on a conductive mandrel (65). The F-Spring (63) as shown in FIG. 6 is a conductive folded beam. Alternatively, a bank or bundle of multiple conductive wires can be attached together and made into the same shape.
All the above electrical contacts have approximately the same performance, although manufacture of the UEC and F-Spring may be simpler at the small size required for chip attach.
The performance advantages of the UEC include:                Elastic compliance of 30% or more of uncompressed height and average contact force in an array 20 gr. per UEC or less.        10 milliohm or less total resistance per UEC in contact with solder bump or contact pad.        10 GHz or more frequency capability.        1 million-touchdown durability.        Service temperature >250 degree C.        High reliability.        Interposer contact pitch to 10 mil or less with I/O to 5000 or more and ends solderable and wire-bondable.        
The possible sizes of these electrical contacts, and the elastic compliance achievable, are the key reasons that these demountable electrical contacts are able to replace BGA, micro-BGA, and flip-chip solder joints and more in high-density packages. The option that the ends of the electrical contacts can be soldered or wire-bonded offers additional convenience in the design of a high-density package.
FIGS. 7a through 7d show a preferred method of manufacturing and design of an interposer using the UEC. An interposer is designed to retain and align an array of demountable electrical contacts for mating with corresponding contact pads on the face of the two components to be connected electrically. It will be understood by one skilled in the art that a similar method might be used with T-spring or F-spring interposers.
Typically the carrier is made of polymer or polymer composite. The electrical contacts are inserted into an array of holes in a required pattern in the carrier. The retention of the electrical contacts is achieved with adhesives or mechanical means or both. Appropriate alignment holes are also drilled in the carrier to align the electrical contact to corresponding contact pads.